Microporous membranes have been used as battery separators in primary and secondary lithium batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, and silver-zinc secondary batteries. The performance of such microporous membranes significantly affects the properties, productivity, and safety of the battery.
Battery separator film is generally produced with a relatively high permeability for the battery's electrolyte. It is desirable for the battery separator film to retain its electrolyte permeability while the battery is exposed to relatively high temperatures, as might be encountered during battery manufacturing, testing, and use, so that the battery does not experience an undue loss of power or capacity.
In most cases, it is desirable for the battery separator film to have a relatively low shutdown temperature (“SDT”) and relatively high meltdown temperature (“MDT”) for improved battery-safety properties, particularly at relatively high battery temperature as might occur as a result of overcharging or rapid discharging. Shut-down is believed to occur when ion mobility through the pores of the membrane is impaired or blocked all-together. Melt-down occurs when the membrane loses its structural integrity, potentially resulting in a short-circuit in the battery.
While improvements have been made, there is still a need for relatively thin microporous membrane useful as battery separator film having an increased MDT while at least maintaining acceptable performance with respect to other desirable properties, such as puncture, shrink, and permeability.